CONCURRENT DETERMINATION OF MULTIFUNCTION AL LIGAND-METAL COMPLEXES FORMATION ENTHALPIES AND CONSTANTS BY COUPLING CALORIMETRY WITH POTENTIOMETRY .5. COMPLEXATION OF COPPER (II) AND NICKEL (II) BY OXIDIZED GLUTATHIONE GLYCYL-METHIONINE AND GLYCYL-GLYCINE

Copper(II) and nickel(II) complexation by oxidized glutathione and by two dipeptides (glycyl-glycin and glycyl-methionin) was carried out by coupling two techniques: pH-metric and calorimetric measurements, at 25-degrees-C and 0.5 mol dm-3 KNO3 medium for the didpeptides and 0.15 KNO3 mol dm-3 medium for oxydized glutathione. The complexes formatio n enthalpies of Cu(II)-dipeptide systems show the influence of the sid e chain of the gly-met dipeptide, when compared with the gly-gly dipep tide. The first four deprotonation enthalpies of oxidized glutathione (pK1, pK2, pK3 and pK4) were determined. The determination of the comp lexe formation enthalpies of NiAH, NiA, Ni2A on the one hand, and CuAH , CuA, Cu2AH on the other hand, was carried out. These enthalpy values are important when compared with the enthalpy formation values of met allic ion-heteroatome bonds. These results suggest the existence of in tramolecular bonds which are favored when the ligand chelates the meta l ion.